1. Technical Field
The present invention relates to magnesium-based alloy wire of high toughness, and to methods of manufacturing such wire. The invention further relates to springs in which the magnesium-based alloy wire is utilized.
2. Description of the Related Art
Magnesium-based alloys, which are lighter than aluminum, and whose specific strength and relative stiffness are superior to steel and aluminum, are employed widely in aircraft parts, in automotive parts, and in the bodies for electronic goods of all sorts.
Nevertheless, the ductility of Mg and alloys thereof is inadequate, and their plastic workability is extremely poor, owing to their hexagonal close-packed crystalline structure. This is why it has been exceedingly difficult to produce wire from Mg and its alloys.
What is more, although circular rods can be produced by hot-rolling and hot-pressing an Mg/Mg alloy casting material, since they lack toughness and their necking-down (reduction in cross-sectional area) rate is less than 15% they have not been suited to, for example, cold-working to make springs. In applications where magnesium-based alloys are used as structural materials, moreover, their YP (tensile yield point) ratio (defined herein as 0.2% proof stress [i.e., offset yield strength]/tensile strength) and torsion yield ratio τ0.2/τmax (ratio of 0.2% offset strength τ0.2 to maximum shear stress τmax in a torsion test) are inferior compared with general structural materials.
Meanwhile, high-strength Mg—Zn—X system (X: Y, Ce, Nd, Pr, Sm, Mm) magnesium-based alloys are disclosed in Japanese Pat. App. Pub. No. H07-3375, and produce strengths of 600 MPa to 726 MPa. The published patent application also discloses carrying out a bend-and-flatten test to evaluate the toughness of the alloys.
The forms of the materials obtained therein nevertheless do not go beyond short, 6-mm diameter, 270-mm length rods, and lengthier wire cannot be produced by the method described (powder extrusion). And because they include addition elements such as Y, La, Ce, Nd, Pr, Sm, Mm on the order of several atomic %, the materials are not only high in cost, but also inferior in recyclability.
In the Journal of Materials Science Letters, 20, 2001, pp. 457-459, furthermore, the fatigue strength in an AZ91 alloy casting material is described, and being on the approximately 20 MPa level, is extremely low.
In Symposium of Presentations at the 72nd National Convention of the Japan Society of Mechanical Engineers, (I), pp. 35-37, results of a rotating-bending fatigue test on material extruded from AZ21 alloy are described, and indicate a fatigue strength of 100 MPa, although the evaluation is not up to 107 cycles. In Summary of Presentations at the 99th Autumn Convention of the Japan Institute of Light Metals (2000), pp. 73-74, furthermore, rotating-bending fatigue characteristics of materials formed by Thixomolding™ AE40, AM60 and ACaSr6350p are described. The fatigue strengths at room temperature are respectively 65 MPa, 90 MPa and 100 MPa, however. In short, as far as rotating-bending fatigue strength of magnesium-based alloys is concerned, fatigue strengths over 100 MPa have not been obtained.